This invention relates to silicon web growth technology. Dendritic web silicon substrates are produced for solar cell manufacturing, by growing thin ribbons of single crystal material from liquid silicon. The ribbon produced typically appears as a thin (100 microns) single crystal structure approximately 5 cm wide, bounded at each vertical edge by a single silicon dendrite with a thickness of approximately 700 microns. In the growth process, the center section is actually a liquid surface tension film supported by the two dendrites which have begun to solidify beneath the surface of the undercooled melt. As the crystal is pulled from the melt surface, the liquid film freezes in a stable, smooth, single crystal state. The process is described in more detail in U.S. Pat. No. 4,389,377 and Seidensticker, Journal of Crystal Growth, 39, 1977, the disclosures of which are hereby incorporated by reference.
Control of temperature and temperature distribution is absolutely critical in this process, requiring stability on the order of 0.1 C. at an absolute temperature of 1410 C. The thicknesses of the edge dendrites are extremely sensitive to temperature, and provide a convenient method of controlling temperature at the crystal based on the size of these dendrites. If the melt temperature is too low, the web degenerates from a single crystal state; if the melt temperature is too high, the crystal pulls away from the melt due to insufficient dendrite growth beneath the melt. Each of these conditions is sufficient reason to terminate the growth of an individual crystal. The throughput of the process and hence the prospect for achieving a low cost process is extremely dependent on average crystal length.
The existing art involves controlling the melt temperature through the use of a pyrometer based temperature controller which controls a single point in the hot zone. An operator adjusts the setpoint of the loop by observing the dendrite thickness of the growing crystal. In addition, the operator adjusts the lateral temperature symmetry of the melt by moving an induction coil relative to the hot zone. The operator makes the adjustments based on the visually perceived difference in the thickness of each of the two edge dendrites. The operator must continually look through a quartz window at a dendrite which is located approximately 50 cm away and make a visual estimate of dendrite edges which are only 0.7 mm thick. The range of thickness control necessary to maintain continuous growth is about +/-0.2 mm. This manual control method is extremely subjective, being based on the operator's vision, and requires a substantial investment of the operator's time, continually observing the crystal.